Fabrication of three-dimensional nanoimprint mold using inorganic resist in low accelerating voltage electron beam lithography

Three-dimensional (3D) resolution of inorganic resist pattern, which was exposed with control of acceleration voltage electron beam lithography (CAV-EBL) in low accelerating voltage was examined. The system can make features with varying developed-depths. Three-dimensional pattern with a few hundred nanometer linewidth was fabricated with a CAV-EBL. The pattern depths on inorganic resist were gradated with 5 nm depth-resolution per 30 V. By controlling the pattern depth, a seven stairs blade-shaped binary optics mold was fabricated, and then a replica pattern of the mold was made by using UV-NIL.     

Large area nanosize array stamp for UV-based nanoimprint lithography fabricated by size reduction process

A novel size reduction process using electron beam lithography (EBL) combining with wet etching technique is developed as a possible solution for producing large area and low cost nanopattern stamp for UV-based nanoimprint lithography (UV-NIL). In the first step, a microstructure stamp with 1.4 μm periodical pore array and aspect ratio of 1:1 was formed over a 1 inch² area on a quartz substrate. This process was carried out using common electron beam lithography (EBL) equipment, which was easily available in the modern integrated circuits (IC) semiconductor factory. Afterwards, with a controlled wet etching technique, the pore array was changed into tip patterns with the line width below 100 nm and the period keeping as before. The uniformities and nanopattern accuracies were investigated to identify its possibility as a UV-NIL stamp by AFM and SEM. Finally, as a demonstration, the as obtained stamp was used as a positive stamp to replicate the nanotips into UV-curable resist successfully by a UV-NIL process. The method developed for the mold of nanoimprint lithography would be a simple and low price approach to fabricate large area UV-NIL stamp and the nanotip array structures would be widely used in two dimensional (2D) photonic crystal application.     

Surface charging suppression using PEDOT/PSS in the fabrication of three dimensional structures on a quartz substrate

Pattern writing on insulating materials (e.g. quartz) using electron beam lithography (EBL) is a challenging task and it is even more difficult when the pattern is three dimensional (3D). Surface charging trapped on insulating substrates may deflect the electron beam during electron beam pattern writing causing undesired effects.In this work, the surface charging has been suppressed by top coating with water soluble conductive polymer layer using poly (3,4-Etylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). The 3D masking profiles are created on a negative tone photoresist (Microresist, ma-N2403) using Raith150 EBL tool with variable dose controlled beam exposure. The 3D patterns have been transferred onto the quartz substrate by single step reactive ion etching (RIE) with suitable resist to substrate selectivity.We have demonstrated the fabrication of 3D geometrical shapes such as pyramids, hemispheres, and cones with dimensions down to 300 nm using this technique without any surface charging effects.     

Residue-free room temperature UV-nanoimprinting of submicron organic thin film transistors

In this study we report on an innovative nanoimprint process for the fabrication of entirely patterned submicron OTFTs in a bottom-gate configuration. The method is based on UV-Nanoimprint Lithography (UV-NIL) combined with a novel imprint resist whose outstanding chemical and physical properties are responsible for the excellent results in structure transfer. In combination with a pretreated stamp the UV-curable resist enables residue-free imprinting thus making etching obsolete. A subsequent lift-off can be done with water. The UV-NIL process implies no extra temperature budget, is time saving due to short curing times, eco-friendly due to a water-based lift-off, simple because it is etch-free and completely r2r compatible. It works perfectly even if ultra-thin organic and hybrid films are used as gate dielectrics. On this basis entirely patterned functional submicron OTFTs with pentacene as the semiconductor are fabricated showing clear saturation, low switch-on voltage (～3 V) and a sufficiently high on–off ratio (10³).     

Fabrication of high density nano-pillar type phase change memory devices using flexible AAO shaped template

In this study, the high density nano-pillar type phase change memory was fabricated using duplicating nano-patterns of the anodic alumina oxide (AAO) by nanoimprint process. The high density nano-hole array of AAO template was transferred to the flexible PVC polymer template using hot embossing method. To use the flexible AAO shaped template for UV-NIL, the high density nano-pillar type Ge₂Sb₂Te₅ patterns were fabricated, and the electrical properties of the device were evaluated by conducting atomic force microscopy, connected electrical measurement system. To use the flexible AAO shaped template for UV-NIL, high density GST pattern could be fabricated even on the flexible polyimide (PI) substrate.     

Direct soft UV-NIL with resist incorporating carbon nanotubes

As a potential candidate for the next generation of nanolithography, nanoimprint lithography (NIL) has drawn ever-increasing worldwide attention. It involves physical contact to overcome the optical limits occurring in sub-100 nm photolithography. Affordable tool cost is one of major attractive points of NIL. This work proposes the idea of incorporating carbon nanotubes (CNTs) in the resin used for ultraviolet nanoimprinting (UV-NIL). CNTs can make the resin electrically conductive when mixed with it. Patterns imprinted in the CNT-mixed resist can then be used to replace conductive metal structures directly. This enhances the productivity of basic UV-NIL where the imprinted patterns are used as sacrificial etch masks. In this work, several types of CNTs were purified chemically and dispersed before being mixed with UV-NIL resin using ultrasonic vibration. On drops of CNT-mixed resin, soft UV-NIL was performed using a polydimethylsiloxane (PDMS) stamp with a minimum feature size in the range of 200 nm. Even with increased resin viscosity due to the addition of CNTs, UV imprinting down to 200 nm was successfully done with moderate pattern fidelity. The loading rate of nanotubes should be minimized to prevent the increased viscosity from degrading the pattern transfer resolution. The electrical conductivity of CNT-mixed resist increases with the loading of CNTs. Therefore, the trade-off between the electrical properties and pattern transfer resolution needs to be optimized carefully.     

新型高抗粘紫外纳米压印光刻胶的工艺研究

为了减少紫外纳米压印技术脱模过程中的接触粘附力,开发了一种新型高流动、抗粘的紫外纳米压印光刻胶。光刻胶以BMA为聚合单体,添加特定配比的交联剂和光引发剂配置而成。紫外纳米压印实验在本课题组自主研发的IL-NP04型纳米压印机上完成。实验得到光刻胶掩膜膜厚为1.21μm,结构尺寸深246nm,周期937.5nm。实验结果表明,在没有对石英模板表面进行修饰的情况下,该光刻胶依然表现出高可靠性和高图形转移分辨率,有效减少了紫外纳米压印工艺中的模板抗粘修饰的工艺步骤。     

Filling behavior of UV nanoimprint resin observed by using a midair structure mold

The filling behavior of resin during UV nanoimprint lithography (UV-NIL) was observed by using a “midair structure mold” and by changing the imprint pressure. The midair structure molds were fabricated by electron beam lithography (EBL) using hydrogen silsesquioxane (HSQ) as a negative tone resist. After the fabrication of midair structure mold, two types of surface treatment molds, which were with or without release coating, were prepared. Using these molds, the filling behavior of a UV curable resin was investigated at various pressures. The results indicate that a pressure of approximately 1.2 MPa is necessary for complete filling in the case of molds treated with a release agent. This method demonstrates effect of a release coating for UV-NIL.     

Fabrication of high aspect ratio nanostructures on 3D surfaces

A combination of different materials and processes was used to create high aspect ratio nanostructures on 3D surfaces. The high aspect ratio structures were formed on thermoplastic foils using UV-Nanoimprintlithography (UV-NIL) with a poly (dimethylsiloxane) (PDMS) stamp which was fabricated by soft lithography. An epoxy mixture with a higher glass transition temperature than the thermoplastic foil was used as a resist for UV-NIL. The hydrophobicity of structured substrates was characterized by the surface contact angle. Substrates with an additional chemical treatment were also produced and characterized. Results of contact angle measurement showed that superhydrophobic surface properties can be obtained with structured and chemically treated samples. The foils were further used as a substrate in a thermoforming process to transfer the structures into a microchannel. Using this process, 3D structured foils can be fabricated with high accuracy. The foils were used as a master structure for a replica molding process which allowed the fabrication of 3D structured polymer parts. With the presented method, microchannels with superhydrophobic surface properties can be fabricated.     

Fabrication of metallic oxide nanowires

Micron length nanowires with varying widths were patterned in half-metallic La_2/3Sr_1/3MnO₃ (LSMO) thin films of different thicknesses, using a thin negative-tone electron beam lithography (EBL) process. Patterns were realized in the high resolution hydrogen silsesquioxane (HSQ) inorganic resist and successfully transferred to the manganite via an energetic argon ion beam etching (IBE). We have obtained wires with widths down to 65 nm and length up to 4 μm that exhibit transport properties comparable with those of unpatterned thin films.     

Nanoholes by soft UV nanoimprint lithography applied to study of membrane proteins

In this paper, we present an alternative technique to the well-known electron beam lithography in order to realize nanoholes in the silicon substrates for biological applications. The used technique is soft UV nanoimprint lithography (UV-NIL). We optimized the fabrication of silicon based supports obtained by soft UV-NIL and reactive ion etching to carry out very large arrays of nanoholes. The resolution limits are investigated when using poly(dimethylsiloxane) as flexible mold material. RIE conditions are initiated to limit the lateral mask resist etch.     

Characterisation of ultraviolet nanoimprint dedicated resists

We have developed a new resist material, named NILTM105, for the purpose of ultraviolet curing nanoimprint lithography. Its capability for micro- and nano-scale features patterning has been experimentally analyzed and compared to two other commercially available UV-NIL resists (AMONIL-MMS4 proposed by AMO GmbH, Germany and PAK-01 from Toyo Gosei, Japan). Using 3D-atomic force microscopy, cross section scanning electron microscopy, CD-SEM and ellipsometric measurements, the suitability of this resist for a reliable replication of the mold features was confirmed. Besides, detailed study of the residual thickness and features height variation as a function of pattern size and density has proven that the three investigated resists can flow over distances on the millimeter range. Finally, the etch resistance of the home-developed material was characterized under several plasmas conditions. It was found out that the etch rates values are compatible with the use of this resist as a masking layer during transfer steps.     

A 4-in.-based single-step UV-NIL tool using a low vacuum environment and additive air pressure

Ultraviolet nanoimprint lithography (UV-NIL) is a promising technology for the fabrication of sub-10-nm features. Research has focused on employing a large-area stamp to improve UV-NIL throughput, but a large-area stamp makes it difficult to obtain an acceptable uniform residual layer thickness and/or avoid defects such as air entrapment. This paper presents the development of a single-step UV-NIL tool in which a 4-in. Pyrex stamp is first used to imprint coated resin against a 4-in. Si wafer in a low vacuum environment. Pressurized N₂ is subsequently applied to the wafer bottom to improve the quality of imprint results. This UV-NIL tool was used to successfully imprint a 4-in. stamp with recessed patterns engraved over the entire stamp areas onto a 4-in. Si wafer.     

A versatile pattern inversion process based on thermal and soft UV nanoimprint lithography techniques

Si master molds are generally patterned by electron-beam lithography (EBL) that is known to be a time-consuming nanopatterning technique. Thus, developing mold duplication process based on high throughput technique such as nanoimprint lithography can be helpful in reducing its fabrication time and cost. Moreover, it could be of interest to get inverted patterns (holes instead of pillars) without changing the master EBL process. In this paper, we propose a two step process based on thermal nanoimprint lithography (T-NIL) (step 1) and soft UV assisted nanoimprint lithography (UV-NIL) (step 2) to invert a master EBL mold. After the two inversion steps, the grand-daughter Si mold exhibits the same pattern polarity as the EBL mold. For step 1, pattern transfer using ion beam etching (IBE) of a thin metallic underlayer is the critical step for dimension control due to the low NXR1020 resistance. For step 2, the optimized reactive ion etching (RIE) step allows transfer with good anisotropy even for nanostructures at the 50 nm-scale. For structures larger than 100 nm, this inversion process has been successfully applied to large field replication (up to 1.5 cm²) on whole wafer.     

Nanopatterned UV curable hydrogels for biomedical applications

With an increasing use of emerging patterning technologies such as UV-NIL in biotechnological applications there is at the same time a raising demand for new material for such applications. Here we present a PEG based precursor mixed with a photoinitiator to make it UV sensitive as a new material aimed at biotechnological applications. Using HSQ patterned quartz stamps we observed excellent pattern replication indicating good flow properties of the resist. We were able to obtain imprints with <20 nm residual layer. The PEG based resist has hydrogel properties and it swelling in water was observed by AFM.     

Electron beam nanolithography in AZnLOF 2020

We present a lithography process using electron beam lithography with an optical resist AZnLOF 2020 for pattern transfer. High-resolution 100 keV electron beam lithography in 400 nm layers of negative resist AZnLOF 2020 diluted 10:4 with PMGEA is realized. After the electron beam lithography process, the resist is used as a mask for reactive ion etching. We performed the transfer of patterns by RIE etching of the substrate allowing a final resolution of 100 nm. We demonstrate the patterning in an insulating layer, thus simplifying the fabrication process of various multilayer devices; proximity correction has been applied to improve pattern quality and also to obtain lines width according to their spacing. This negative resist is removed by wet etching or dry etching, could allow combining pattern for smallest size down to 100 nm by EBL techniques and for larger sizes by traditional lithography using photomask.     

A step-and-repeat UV-nanoimprint lithography process using an elementwise patterned stamp

Ultraviolet-nanoimprint lithography (UV-NIL) is a promising cost-effective method for defining nanoscale structures at room temperature and low pressure. To apply a large-area stamp to a high throughput step-and-repeat process at atmospheric conditions, we proposed a new UV-NIL process that uses an elementwise patterned stamp (EPS), which consists of elements separated by channels, and additive gas pressurization. The proposed UV-NIL process required just four imprints to press an 8-in. wafer. EPS features measuring 50-80 nm were successfully transferred onto the wafers. The experiments demonstrated that a 5 × 5-in.² EPS could be used with a step-and-repeat UV-NIL process to imprint 8-in. wafers under atmospheric conditions.     

CO2-Based Dual-Tone Resists for Electron Beam Lithography

The increasing global environmental and energy crisis has urgently motivated the advancement of sustainable materials. Significant effort has been focused on developing new materials to replace the fossil-based resists in the semiconductor industry based on greener sources such as ice, dry ice, small organic molecules, and proteins. Such resist materials, however, have yet to meet the stringent requirements of high sensitivity, high resolution, reliable repeatability, and good compatibility with the current protocols. To this end, CO₂-based polycarbonates (CO₂-PCs) obtained from the copolymerization of CO₂ and epoxides are demonstrated as sustainable dual-tone (positive & negative tone) resists for electron beam lithography. By adjusting the chemical structure, developing agent, and molecular weight, the CO₂-PCs present high sensitivities to electron beam (1.3/120 µ C cm⁻²), narrow critical dimensions (29/58 nm), and moderate line edge roughness (4.6/26.7 nm) for negative and positive resists, respectively. A deep understanding of the exposure mechanism of CO₂-PC resists is provided on the basis of the Fourier transform infrared, Raman, and electron ionization mass spectroscopy. 2D photonic crystal devices are fabricated using the negative and positive CO₂-PC resists, respectively, and both devices show distinct colors derived from their well-defined nanostructures, indicating the great practical potential of CO₂-derived electron beam resists.     

XPS study of the degradation mechanism of fluorinated anti-sticking treatments used in UV nanoimprint lithography

Thanks to their low surface energy, fluorinated anti-sticking layers are widely used in UV nanoimprint lithography (UV-NIL) to treat the mold and facilitate its separation from the imprinted resist. However, it has been reported that release properties of the stamp deteriorate with repeated imprint operations. In this paper, X-ray photoelectron spectroscopy is used to study the mechanism of the fluorinated treatment degradation. A specific experimental protocol is used in order to avoid further degradation under X-ray exposure. It has been observed that a large amount of fluorinated molecules are removed in the first imprint steps and deposited on the surface of the imprinted resist. After this first stage, we observed that fluorinated molecules are progressively degraded along their chain during the NIL process.     

Nanoprint lithography of gold nanopatterns on polyethylene terephthalate

Nano-order metal pattern printing on plastic substrates was established by using hard stamp nanoprint lithography (NPL). A spin-on-glass (SOG) material, which is almost the same as quartz in composition, was used as the material for the hard stamp. The SOG acted as a positive-tone electron beam (EB) resist. Nanopatterns were fabricated by using electron beam lithography (EBL), and a developed pattern of SOG was used as the hard stamp. Further, two types of release coating methods were utilized. One method used a conventional silan coupling agent and the other, a chromium layer. After comparing the results of the methods, we found that the chromium layer formed a smooth surface and therefore used this layer as the release layer. In addition, chromium was changed to Cr₂O₃ because of the exposure to atmospheric air. Gold was used as the transfer metal and was deposited on the hard stamp covered with the chromium release layer. This stamp was then placed in contact with a PET substrate at 80 °C for 30 min. A gap width of less than 30 nm of gold was transferred onto the PET substrate. This process is very simple, and yet, it makes it possible to obtain a very high resolution metal pattern transfer by using hard stamp NPL.